Zoom lens system and electronic imaging apparatus using the same

ABSTRACT

A zoom lens system includes a negative first lens group, a positive second lens group, and a negative third lens group, in that order from the object side, wherein upon zooming from the short focal length extremity to the long focal length extremity, the distance between the first lens group and the second lens group decreases. The third lens group includes a negative lens element having a concave surface on the image side, a positive lens element having a convex surface on the image side, and a negative lens element having convex surface on the image side, in that order from the object side. An electronic imaging apparatus using this zoom lens system is also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a zoom lens system and an electronic imaging apparatus using the same.

2. Description of Related Art

In the digital camera market, while there is a tendency for further miniaturization and a higher zoom ratio, there also is an increased demand for a higher definition (higher picture quality). One way to obtain a higher definition is to, for example, use an image sensor (image pickup device) that has a large APS size. In an SLR camera, the image sensor is large, so that a high definition image can be achieved, however, since the overall camera system is large and heavy, demands for further miniaturization cannot be met. In recent years, a so-called “mirrorless” SLR camera has been proposed in which the quick-return mirror, which is a major characteristic feature of SLR cameras, is removed and does not use an optical finder system; however, it cannot be said that such mirrorless SLR cameras achieve sufficient miniaturization. Furthermore, lens-shutter zoom lens systems which pursue miniaturization have also been proposed, however, the incidence-angle of light that is incident on the imaging surface is not sufficiently perpendicular to the imaging surface (i.e., has poor telecentricity), so that use of such lens-shutter zoom lens systems with a digital imaging sensor is not realistic.

Conventionally, a zoom lens system is known in the art which has three lens groups, i.e., a negative first lens group, a positive second lens group, and a negative third lens group, in that order from the object side, for use with an image sensor having a large APS size. In this conventional zoom lens system, it is typical for the third lens group to be configured of two lens elements, i.e., a negative lens element and a positive lens element, in that order from the object side, or a positive lens element and a negative lens element, in that order from the object side. Examples of the latter arrangement of the third lens group can be found in Japanese Unexamined Patent Publication Nos. 2001-290076, 2002-221660 and 2009-25534.

However, although the former zoom lens system in which the third lens group is configured of a negative lens element and a positive lens element, in that order from the object side, is advantageous for improving telecentricity, various aberrations such as lateral chromatic aberration, astigmatism, distortion and spherical aberration that occur in the negative first lens group cannot be favorable corrected. Furthermore, since the negative lens element on the object side within the third lens group and the positive lens element on the image side within the third lens group constitute a local retrofocus lens arrangement, an increase in the overall length of the zoom lens system cannot be avoided.

Whereas, in the latter zoom lens system in which the third lens group is configured of a positive lens element and a negative lens element, in that order from the object side, in order to secure the negative refractive power of the third lens group, the refractive power of the negative lens element on the image side needs to be increased, so that the refractive-power balance between the positive lens element on the object side and the negative lens element on the image side is not favorable.

SUMMARY OF THE INVENTION

In view of the above-described problems, the present invention provides a zoom lens system configured of three lens groups, i.e., a negative lens group, a positive lens group and a negative lens group, in that order from the object side, which uses, e.g., a large APS-sized image sensor while achieving compact zoom lens system that has a superior optical quality; furthermore, the present invention also provides an electronic imaging apparatus which utilizes such a zoom lens system.

According to an aspect of the present invention, a zoom lens system is provided, including a negative first lens group, a positive second lens group, and a negative third lens group, in that order from the object side, wherein upon zooming from the short focal length extremity to the long focal length extremity, the distance between the first lens group and the second lens group decreases. The third lens group includes a negative lens element having a concave surface on the image side, a positive lens element having a convex surface on the image side, and a negative lens element having convex surface on the image side, in that order from the object side.

It is desirable for the following condition (1) to be satisfied: 0.8<f3a/f3c<3.0  (1), wherein f3a designates the focal length of the negative lens element which is provided closest to the object side within the third lens group, and f3 c designates the focal length of the negative lens element which is provided closest to the image side within the third lens group.

It is desirable for the third lens group to include a focusing lens group which is moved along the optical axis direction during focusing, wherein the following condition (2) is satisfied: 0.65<Z3/Z<0.80  (2), wherein

Z3=m3 t/m3 w,

Z=ft/fw,

m3 t designates the lateral magnification of the third lens group at the long focal length extremity when focused on an object at infinity, m3 w designates the lateral magnification of the third lens group at the short focal length extremity when focused on an object at infinity, ft designates the focal length of the entire the zoom lens system at the long focal length extremity, and fw designates the focal length of the entire the zoom lens system at the short focal length extremity.

It is desirable for the following conditions (3) and (4) to be satisfied: 1.0<f1/f3<2.5  (3), and −0.90<f2/f3<−0.60  (4),wherein f1 designates the focal length of the first lens group, f2 designates the focal length of the second lens group, and f3 designates the focal length of the third lens group.

In an embodiment, an electronic imaging apparatus is provided, including the above-described zoom lens system, and an image sensor which converts an image formed by the zoom lens system into electrical signals.

According to the present invention, a zoom lens system configured of three lens groups, i.e., a negative lens group, a positive lens group and a negative lens group, in that order from the object side, is achieved which uses, e.g., a large APS-sized image sensor while achieving compact zoom lens system that has a superior optical quality; furthermore, the present invention also provides an electronic imaging apparatus which utilizes such a zoom lens system.

The present disclosure relates to subject matter contained in Japanese Patent Application No. 2010-256911 (filed on Nov. 17, 2010) which is expressly incorporated herein in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be discussed below in detail with reference to the accompanying drawings, in which:

FIG. 1 shows a lens arrangement of a first numerical embodiment of a zoom lens system, according to the present invention, at the long focal length extremity when focused on an object at infinity;

FIGS. 2A, 2B, 2C and 2D show various aberrations that occurred in the lens arrangement shown in FIG. 1;

FIG. 3 shows a lens arrangement of the first numerical embodiment of the zoom lens system, according to the present invention, at the short focal length extremity when focused on an object at infinity;

FIGS. 4A, 4B, 4C and 4D show various aberrations that occurred in the lens arrangement shown in FIG. 3;

FIG. 5 shows a lens arrangement of a second numerical embodiment of a zoom lens system, according to the present invention, at the long focal length extremity when focused on an object at infinity;

FIGS. 6A, 6B, 6C and 6D show various aberrations that occurred in the lens arrangement shown in FIG. 5;

FIG. 7 shows a lens arrangement of the second numerical embodiment of the zoom lens system, according to the present invention, at the short focal length extremity when focused on an object at infinity;

FIGS. 8A, 8B, 8C and 8D show various aberrations that occurred in the lens arrangement shown in FIG. 7;

FIG. 9 shows a lens arrangement of a third numerical embodiment of a zoom lens system, according to the present invention, at the long focal length extremity when focused on an object at infinity;

FIGS. 10A, 10B, 10C and 10D show various aberrations that occurred in the lens arrangement shown in FIG. 9;

FIG. 11 shows a lens arrangement of the third numerical embodiment of the zoom lens system, according to the present invention, at the short focal length extremity when focused on an object at infinity;

FIGS. 12A, 12B, 12C and 12D show various aberrations that occurred in the lens arrangement shown in FIG. 11; and

FIG. 13 shows a zoom path of the zoom lens system according to the present invention.

DESCRIPTION OF THE EMBODIMENTS

The zoom lens system according to the present invention, as shown in the zoom path of FIG. 13, is configured of a negative first lens group G1, a positive second lens group G2 and a negative third lens group G3, in that order from the object side. A diaphragm S which is provided in between the first lens group G1 and the second lens group G2 moves integrally with the second lens group G2 during zooming. ‘I’ designates the imaging plane. The third lens group G3 constitutes a focusing lens group which is moved during a focusing operation (the third lens group G3 is advanced toward the image side upon carrying out a focusing operation on an object at infinity to an object at a finite distance).

The zoom lens system, upon zooming from the short focal length extremity (WIDE) to the long focal length extremity (TELE), moves the first through third lens groups G1 through G3 in the optical axis direction while reducing the distance between the first and second lens groups G1 and G2, and reducing the distance between the second and third lens groups G2 and G3.

More specifically, as shown in the zoom path of FIG. 13, in each of the first through third numerical embodiments, upon zooming from the short focal length extremity to the long focal length extremity, the first through third lens groups G1 through G3 each move monotonically toward the object side.

In each of the first through third numerical embodiments, the first lens group G1 is configured of a negative lens element 11 and a positive lens element 12, in that order from the object side.

In each of the first through third numerical embodiments, the second lens group G2 is configured of a cemented lens formed from a positive lens element 21 and a negative lens element 22; and a positive lens element 23, in that order from the object side. The positive lens element 23 has an aspherical surface on each side thereof.

In each of the first through third numerical embodiments, the third lens group G3 is configured of a negative lens element (a negative lens element having a concave surface on the image side) 31, a positive lens element (a positive lens element having a convex surface on the image side) 32, and a negative lens element (a negative lens element having a convex surface on the image side) 33, in that order from the object side. In each of the first through third numerical embodiments, the negative lens element 31 has an aspherical surface on the object side thereof. In the first and second numerical embodiments, the positive lens element 32 has an aspherical surface on both sides thereof, whereas in the third numerical embodiment, the positive lens element 32 has an aspherical surface only on the image side thereof.

The zoom lens system of the present invention constitutes a retrofocus lens system at the short focal length extremity having a negative front lens group (first lens group G1) and a positive rear lens group (second and third lens groups G2 and G3), thereby improving telecentricity of the zoom lens system; and constitutes a telephoto lens system at the long focal length extremity having a positive front lens group (first and second lens groups G1 and G2) and a negative rear lens group (third lens group G3), thereby reducing the overall length of the zoom lens system. According to this lens arrangement, both an improvement in the telecentricity and a reduced overall length of the zoom lens system can both be achieved.

In order to improve the telecentricity of the zoom lens system, it is appropriate to configure the negative third lens group G3 of a negative lens element and a positive lens element, in that order from the object side. However, if the third lens group G3 is only configured of two lens elements, i.e., a negative lens element and a positive lens element, in that order from the object side, the various aberrations that occur in the first lens group G1 such as lateral chromatic aberration, astigmatism, distortion and spherical aberration, etc., cannot be favorably corrected.

Therefore, in the zoom lens system according to the present invention, by providing a negative lens element (33) closest to the image side within the third lens group G3, and configuring the third lens group G3 so as to have three lens elements, i.e., a negative lens element, a positive lens element and a negative lens element, in that order from the object side, the telecentricity can be improved while the various remaining aberrations that occurred at the first lens group G1 can be favorably corrected by the third lens group G3. Furthermore, the effect of such an arrangement is most apparent when the third lens group G3 is configured of the following three lens elements: a negative lens element having a concave surface on the image side, a positive lens element having a convex surface on the image side, and a negative lens element having a convex surface on the image side, in that order from the object side, as in the numerical embodiments of the zoom lens system.

Condition (1) specifies the ratio of the focal length of the negative lens element 31 provided closest to the object side within the third lens group G3 to the focal length of the negative lens element 33 provided closest to the image side within the third lens group G3, and is for improving the telecentricity of the zoom lens system while also correcting the various aberrations that occurred at the first lens group G1.

If the upper limit of condition (1) is exceeded, the refractive power of the negative lens element 33 that is provided closest to the image side within the third lens group G3 becomes too strong, which is advantageous for correcting various aberrations that occurred at the first lens group G1, however, it becomes difficult to maintain telecentricity.

If the lower limit of condition (1) is exceeded, the refractive power of the negative lens element 31 that is provided closest to the object side within the third lens group G3 becomes too strong, which is advantageous for improving telecentricity, however, it becomes difficult to correct the various aberrations that occurred at the first lens group G1.

As described above, the third lens group G3 constitutes a focusing lens group which is moved along the optical axis during a focusing operation.

With respect to the above-described lens arrangement, condition (2) specifies the proportion of the ratio of the lateral magnification of the third lens group G3 at the long focal length extremity when focused on an object at infinity to the lateral magnification of the third lens group G3 at the short focal length extremity when focused on an object at infinity (i.e., the zoom ratio of the third lens group G3), to the ratio of the focal length of the entire zoom lens system, at the long focal length extremity to the focal length of the entire zoom lens system at the short focal length extremity. By satisfying condition (2), a focal shift that would otherwise occur during zooming can be prevented. Namely, by satisfying condition (2), the change in the zoom ratio of the entire zoom lens system can be made to coincide with the change of focusing sensitivity of the third lens group G3, which constitutes a focusing lens group, so that a focal shift which would otherwise occur during zooming can be prevented.

If either the upper or lower limits of condition (2) is exceeded, the focal shift during zooming increases.

Condition (3) specifies the ratio of the focal length of the first lens group G1 to the focal length of the third lens group G3, and is for achieving both an improvement of telecentricity and further miniaturization of the zoom lens system.

If the upper limit of condition (3) is exceeded, the refractive power of the third lens group G3 becomes too strong, which is advantageous for miniaturization, however, the telecentricity undesirably deteriorates.

If the lower limit of condition (3) is exceeded, the refractive power of the third lens group G3 becomes too weak, so that sufficient miniaturization cannot be achieved.

Condition (4) specifies the ratio of the focal length of the second lens group G2 to the focal length of the third lens group G3, and is for both correcting aberrations and improving telecentricity.

If the upper limit of condition (4) is exceeded, the refractive power of the second lens group G2 becomes too strong, so that it becomes difficult to correct spherical aberration and coma, especially at the long focal length extremity.

If the lower limit of condition (4) is exceeded, the refractive power of the third lens group G3 becomes too strong, so that the telecentricity undesirably deteriorates.

Specific numerical embodiments will be herein discussed. The following numerical embodiments are applied to a zoom lens system used in a digital camera. In the aberration diagrams and the tables, the d-line, the g-line and the C-line show aberrations at their respective wave-lengths; S designates the sagittal image, M designates the meridional image, FNO. designates the f-number, f designates the focal length of the entire optical system, W designates the half angle of view (°), Y designates the image height, fB designates the backfocus, L designates the overall length of the lens system, r designates the radius of curvature, d designates the lens thickness or distance between lenses, N(d) designates the refractive index at the d-line, and vd designates the Abbe number with respect to the d-line. The values for the f-number, the focal length, the half angle-of-view, the image height, the backfocus, the overall length of the lens system, and the distance between lenses (which changes during zooming) are shown in the following order: short focal length extremity, intermediate focal length, and long focal length extremity.

An aspherical surface which is rotationally symmetrical about the optical axis is defined as: x=cy ²/(1+[1−{1+K}c ² y ²]^(1/2))+A4y ⁴ +A6y ⁶ +A8y ⁸ +A10y ¹⁰ +A12y ¹² . . . . wherein ‘x’ designates a distance from a tangent plane of the aspherical vertex, ‘c’ designates the curvature (1/r) of the aspherical vertex, ‘y’ designates the distance from the optical axis, ‘K’ designates the conic coefficient, A4 designates a fourth-order aspherical coefficient, A6 designates a sixth-order aspherical coefficient, A8 designates an eighth-order aspherical coefficient, A10 designates a tenth-order aspherical coefficient, and A12 designates a twelfth-order aspherical coefficient. [Numerical Embodiment 1]

FIGS. 1 through 4D and Tables 1 through 4 show a first numerical embodiment of a zoom lens system according to the present invention. FIG. 1 shows a lens arrangement of the first numerical embodiment of the zoom lens system at the long focal length extremity when focused on an object at infinity. FIGS. 2A, 2B, 2C and 2D show various aberrations that occurred in the lens arrangement shown in FIG. 1. FIG. 3 shows a lens arrangement of the first numerical embodiment of the zoom lens system at the short focal length extremity when focused on an object at infinity. FIGS. 4A, 4B, 4C and 4D show various aberrations that occurred in the lens arrangement shown in FIG. 3. Table 1 shows the lens surface data, Table 2 shows various zoom lens system data, Table 3 shows the aspherical surface data, and Table 4 shows the lens group data of the zoom lens system according to the first numerical embodiment.

The zoom lens system of the first numerical embodiment is configured of a negative first lens group G1, a positive second lens group G2 and a negative third lens group G3, in that order from the object side. The third lens group G3 constitutes a focusing lens group that is moved along the optical axis direction during a focusing operation (the third lens group G3 advances toward the image side when performing a focusing operation while focusing on an object at infinity to an object at a finite distance).

The first lens group G1 (surface Nos. 1 through 4) is configured of a biconcave negative lens element 11 and a positive meniscus lens element 12 having a convex surface on the object side, in that order from the object side.

The second lens group G2 (surface Nos. 6 through 10) is configured of a cemented lens formed from a biconvex positive lens element 21 and a negative meniscus lens element 22 having a convex surface on the image side; and a biconvex positive lens element 23, in that order from the object side. The biconvex positive lens element 23 has an aspherical surface on each side thereof. A diaphragm S (surface No. 5), which is provided in between the first lens group G1 and the second lens group G2, integrally moves with the second lens group G2 along the optical axis during zooming.

The third lens group G3 (surface Nos. 11 through 16) is configured of a biconcave negative lens element (a negative lens element having a concave surface on the image side) 31, a positive meniscus lens element (a positive lens element having a convex surface on the image side) 32 having a convex surface on the image side, and a negative meniscus lens element (a negative lens element having a convex surface on the image side) 33 having a convex surface on the image side, in that order from the object side. The biconcave negative lens element 31 has an aspherical surface on the object side. The positive meniscus lens element 32 has an aspherical surface on each side thereof. An optical filter OP (surface Nos. 17 and 18) is disposed behind (between the third lens group G3 and the imaging plane I) the third lens group G3 (the negative meniscus lens element 33).

TABLE 1 SURFACE DATA Surf. No. r d Nd νd  1 −1334.671 1.200 1.61800 63.4  2 12.871 3.300  3 16.395 2.200 1.84666 23.8  4 25.167 d4  5(Diaphragm) ∞ 1.000  6 14.140 4.215 1.49700 81.6  7 −15.047 1.100 1.90366 31.3  8 −75.813 2.834  9* 29.029 3.823 1.59201 67.0 10* −17.340 d10 11* −104.029 1.500 1.52538 56.3 12 25.683 1.215 13* −54.738 2.360 1.63548 23.9 14* −28.914 4.225 15 −8.870 1.200 1.58913 61.2 16 −19.156 d16 17 ∞ 2.000 1.51633 64.1 18 ∞ — The asterisk (*) designates an aspherical surface which is rotationally symmetrical with respect to the optical axis.

TABLE 2 ZOOM LENS SYSTEM DATA Zoom Ratio 2.85 Short Long Focal Length Intermediate Focal Length Extremity Focal Length Extremity FNO. 3.6 4.9 6.7 f 18.60 28.01 53.00 W 43.0 28.2 15.1 Y 14.24 14.24 14.24 fB 3.00 3.00 3.00 L 59.72 61.65 73.30 d4 16.072 10.253 2.000 d10 5.471 3.178 1.354 d16 3.003 13.049 34.774

TABLE 3 Aspherical Surface Data (the aspherical surface coefficients not indicated are zero (0.00)): Surf. No. K A4 A6 A8 9 0.000 −0.3003E−04   0.5680E−06 0.6042E−08 10 0.000 0.1112E−03 0.3802E−06 0.8883E−08 11 0.000 0.5288E−04 −0.4674E−07   13 0.000 0.1021E−03 0.2720E−05 −0.6838E−08   14 0.000 0.5377E−04 0.1964E−05 0.6335E−08

TABLE 4 LENS GROUP DATA Lens Group 1^(st) Surf. Focal Length 1 1 −36.83 2 6 15.68 3 11 −19.04 [Numerical Embodiment 2]

FIGS. 5 through 8D and Tables 5 through 8 show a second numerical embodiment of a zoom lens system according to the present invention. FIG. 5 shows a lens arrangement of the second numerical embodiment of the zoom lens system at the long focal length extremity when focused on an object at infinity. FIGS. 6A, 6B, 6C and 6D show various aberrations that occurred in the lens arrangement shown in FIG. 5. FIG. 7 shows a lens arrangement of the second numerical embodiment of the zoom lens system at the short focal length extremity when focused on an object at infinity. FIGS. 8A, 8B, 8C and 8D show various aberrations that occurred in the lens arrangement shown in FIG. 7. Table 5 shows the lens surface data, Table 6 shows various zoom lens system data, Table 7 shows the aspherical surface data, and Table 8 shows the lens group data of the zoom lens system according to the second numerical embodiment.

The lens arrangement of the second numerical embodiment is the same as that of the first numerical embodiment except for the following points:

-   (1) The positive lens element 23 of the second lens group G2 is a     positive meniscus lens element having a convex surface on the image     side. -   (2) The negative lens element 31 of the third lens group G3 is a     negative meniscus lens element having a convex surface on the object     side.

TABLE 5 SURFACE DATA Surf. No. r d Nd νd  1 −617.730 1.200 1.61800 63.4  2 13.287 3.300  3 16.041 2.200 1.84666 23.8  4 23.384 d4  5(Diaphragm) ∞ 1.000  6 13.168 4.600 1.49700 81.6  7 −13.157 1.100 1.90366 31.3  8 −42.053 1.771  9* −113.546 4.460 1.59201 67.0 10* −12.046 d10 11* 176.416 1.500 1.52538 56.3 12 20.741 1.610 13* −79.945 2.360 1.60641 27.2 14* −29.955 3.830 15 −8.993 1.200 1.58913 61.2 16 −21.985 d16 17 ∞ 2.000 1.51633 64.1 18 ∞ — The asterisk (*) designates an aspherical surface which is rotationally symmetrical with respect to the optical axis.

TABLE 6 ZOOM LENS SYSTEM DATA Zoom Ratio 2.85 Short Long Focal Length Intermediate Focal Length Extremity Focal Length Extremity FNO. 3.6 4.8 6.7 f 18.60 28.01 53.00 W 43.0 28.3 15.1 Y 14.24 14.24 14.24 fB 4.00 4.00 4.00 L 60.13 61.46 73.25 d4 16.070 9.971 2.000 d10 5.746 3.401 1.394 d16 2.178 11.960 33.728

TABLE 7 Aspherical Surface Data (the aspherical surface coefficients not indicated are zero (0.00)): Surf. No. K A4 A6 A8 9 0.000 −0.1929E−03   −0.9473E−06   10 0.000 0.1392E−04 −0.3276E−06   11 0.000 0.2739E−04 0.1642E−06 13 0.000 0.4397E−04 0.1392E−05 0.6448E−08 14 0.000 −0.2479E−04   0.7396E−06 0.1127E−07

TABLE 8 LENS GROUP DATA Lens Group 1^(st) Surf. Focal Length 1 1 −36.01 2 6 16.09 3 11 −20.16 [Numerical Embodiment 3]

FIGS. 9 through 12D and Tables 9 through 12 show a third numerical embodiment of a zoom lens system according to the present invention. FIG. 9 shows a lens arrangement of the third numerical embodiment of the zoom lens system at the long focal length extremity when focused on an object at infinity. FIGS. 10A, 10B, 10C and 10D show various aberrations that occurred in the lens arrangement shown in FIG. 9. FIG. 11 shows a lens arrangement of the third numerical embodiment of the zoom lens system at the short focal length extremity when focused on an object at infinity. FIGS. 12A, 12B, 12C and 12D show various aberrations that occurred in the lens arrangement shown in FIG. 11. Table 9 shows the lens surface data, Table 10 shows various zoom lens system data, Table 11 shows the aspherical surface data, and Table 12 shows the lens group data of the zoom lens system according to the third numerical embodiment.

The lens arrangement of the third numerical embodiment is the same as that of the first numerical embodiment except for the following points:

-   -   (1) The negative lens element 11 of the first lens group G1 is a         negative meniscus lens element having a convex surface on the         object side.     -   (2) The negative lens element 31 of the third lens group G3 is a         negative meniscus lens element having a convex surface on the         object side.     -   (3) The positive meniscus lens element 32 of the third lens         group G3 has an aspherical surface only on the image side.

TABLE 9 SURFACE DATA Surf. No. r d Nd νd  1 436.154 1.200 1.61800 63.4  2 12.090 3.223  3 14.679 2.275 1.84666 23.8  4 21.358 d4  5(Diaphragm) ∞ 2.550  6 15.119 5.946 1.49700 81.6  7 −12.709 3.801 1.90366 31.3  8 −42.498 1.130  9* 58.643 3.725 1.59201 67.0 10* −16.736 d10 11* 35.815 1.500 1.52538 56.3 12 15.480 3.177 13 −37.301 2.498 1.60641 27.2 14* −20.712 2.579 15 −10.047 1.200 1.60311 60.7 16 −26.977 d16 17 ∞ 2.000 1.51633 64.1 18 ∞ — The asterisk (*) designates an aspherical surface which is rotationally symmetrical with respect to the optical axis.

TABLE 10 ZOOM LENS SYSTEM DATA Zoom Ratio 2.85 Short Long Focal Length Intermediate Focal Length Extremity Focal Length Extremity FNO. 3.6 4.9 6.7 f 18.60 28.00 52.99 W 43.0 28.5 15.2 Y 14.24 14.24 14.24 fB 1.00 1.00 1.00 L 62.41 65.34 81.45 d4 13.529 8.613 2.000 d10 7.464 4.095 1.000 d16 3.617 14.826 40.651

TABLE 11 Aspherical Surface Data (the aspherical surface coefficients not indicated are zero (0.00)): Surf. No. K A4 A6 A8 9 0.000 −0.6450E−04   0.3340E−06 10 0.000 0.3484E−04 0.3835E−06 11 0.000 0.2090E−04 0.3044E−06 14 0.000 −0.5136E−04   −0.2850E−06   −0.3297E−09

TABLE 12 LENS GROUP DATA Lens Group 1^(st) Surf. Focal Length 1 1 −35.84 2 6 17.07 3 11 −22.63

The numerical values of each condition for each embodiment are shown in Table 13.

TABLE 13 Embod. 1 Embod. 2 Embod. 3 Cond. (1) 1.33 1.68 1.95 Cond. (2) 0.72 0.71 0.75 Cond. (3) 1.93 1.79 1.58 Cond. (4) −0.82 −0.80 −0.75

As can be understood from Table 13, the first through third numerical embodiments satisfy conditions (1) through (4). Furthermore, as can be understood from the aberration diagrams, the various aberrations are suitably corrected.

Obvious changes may be made in the specific embodiments of the present invention described herein, such modifications being within the spirit and scope of the invention claimed. It is indicated that all matter contained herein is illustrative and does not limit the scope of the present invention. 

What is claimed is:
 1. A zoom lens system comprising a negative first lens group, a positive second lens group, and a negative third lens group, in that order from the object side, wherein upon zooming from the short focal length extremity to the long focal length extremity, the distance between said first lens group and said second lens group decreases, wherein said third lens group includes a negative lens element having a concave surface on the image side, a positive lens element having a convex surface on the image side, and a negative lens element having convex surface on the image side, in that order from the object side.
 2. The zoom lens system according to claim 1, wherein the following condition (1) is satisfied: 0.8<f3a/f3c<3.0  (1),wherein f3 a designates the focal length of the negative lens element which is provided closest to the object side within said third lens group, and f3 c designates the focal length of the negative lens element which is provided closest to the image side within said third lens group.
 3. The zoom lens system according to claim 1, wherein said third lens group comprises a focusing lens group which is moved along the optical axis direction during focusing, wherein the following condition (2) is satisfied: 0.65<Z3/Z<0.80  (2),wherein Z3=m3 t/m3 w, Z=ft/fw, m3 t designates the lateral magnification of said third lens group at the long focal length extremity when focused on an object at infinity, m3 w designates the lateral magnification of said third lens group at the short focal length extremity when focused on an object at infinity, ft designates the focal length of the entire said zoom lens system at the long focal length extremity, and fw designates the focal length of the entire said zoom lens system at the short focal length extremity.
 4. The zoom lens system according to claim 1, wherein the following conditions (3) and (4) are satisfied: 1.0<f1/f3<2.5  (3), and −0.90<f2/f3<−0.60  (4),wherein f1 designates the focal length of the first lens group, f2 designates the focal length of the second lens group, and f3 designates the focal length of the third lens group.
 5. An electronic imaging apparatus comprising the zoom lens system according to claim 1, and an image sensor which converts an image formed by said zoom lens system into electrical signals. 